Hi again, DH said that our friends should pay their 2kw system off within 5 years. Basically, the bigger the system, the bigger the savings, so it will depend on what size you go for.

Energy prices are tipped to double within the next few years, so you can project that into your calcuations too. This will increase your savings even more.

ETA - Not sure about insurance, but they are meant to be hail resistant. Might be worth calling your insurer to check